If you tailgate someone you can get that magical reduction in air resistance which can increase fuel economy. I hate driving on busy roads for this reason, some jack hole will always be right on my ass and I can't even see their headlights.
I don't doubt that we may someday achieve more autonomous highway cruising where vehicles take 'lead' and 'follow' positions within feet of each other to benefit from drafting, but humans doing it scares me senseless.